doublevr
That's a really bad idea since downvotes are completely arbitrary. It would also be tyranny of the majority actually and the opposite of a place that values free speech. Maybe I have a very unique opinion on a certain topic and I voice it in a perfectly respectful way. So I should have every right to post that opinion and everybody else should be able to see it. I don't care about whether it gets 50 downvotes, that might happen to unpopular opinions. But like you said, we should value and defend differing opinions if they are voiced in a respectful manner.
If you start hiding comments based on downvotes you can just close down the whole comment section, it will become completely pointless, a pure echo-chamber of the majority.